Voltammetric Reduction ofα- andγ*-[S2W18O62]4-and α-, β-, and γ-[SiW12O40]4-: Isomeric Dependence of Reversible Potentials of Polyoxometalate Anions Using Data Obtained by Novel Dissolution and Conventional Solution-Phase Processes
- 19 November 2004
- journal article
- research article
- Published by American Chemical Society (ACS) in Inorganic Chemistry
- Vol. 43 (26) , 8263-8271
- https://doi.org/10.1021/ic049043x
Abstract
Comparative studies on the voltammetric reduction of the α and γ* isomers of Dawson [S2W18O62]4- and α, β, and γ forms of Keggin [SiW12O40]4- polyoxometalate anions have been undertaken. For the six reversible one-electron [S2W18O62]4-/5-/6-/7-/8-/9-/10- processes in acetonitrile, reversible potentials (E0‘) were found to be independent of isomeric form within experimental error (±5 mV). However, because both the α and γ* isomers of [Bu4N]4[S2W18O62] are insoluble in water, solid-state voltammetric studies with microcrystals adhered to electrode surfaces in contact with aqueous Et4NCl and Bu4NCl electrolyte media were also possible. Although no isomeric distinction was again detected in the solid-state studies, it was found that reduction of adhered solid by four or more electron equivalents led to rapid dissolution. When Et4NCl was the electrolyte, this dissolution process coupled with potential cycling experiments enabled conventional solution-phase data to be obtained in water for the analogous six one-electron reduction steps previously detected in acetonitrile. A strong medium effect attributed to Lewis acidity effects was apparent upon comparison with E0‘ data obtained in water and acetonitrile. In contrast, with the [SiW12O40]4- system, E0‘ values for the [SiW12O40]4-/5-/6-/7- processes in acetonitrile exhibited a larger (about 70 mV) dependence on isomeric form, and the isomerization step, [γ-SiW12O40]6-→ [α-SiW12O40]6-, was detected on the voltammetric time scale. The influence of isomeric form on reversible potential data is considered in terms of structural and charge density differences exhibited in the [S2W18O62]4- and [SiW12O40]4- systems studied in this paper and published data available on the α, β, γ, and γ* isomers of [As2W18O62]6- and [P2W18O62]6- Dawson anions and Keggin systems.Keywords
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